Oxide interface displaying electronically controllable ferromagnetism

ABSTRACT

A structure includes an electronically controllable ferromagnetic oxide structure that includes at least three layers. The first layer comprises STO. The second layer has a thickness of at least about 3 unit cells, said thickness being in a direction substantially perpendicular to the interface between the first and second layers. The third layer is in contact with either the first layer or the second layer or both, and is capable of altering the charge carrier density at the interface between the first layer and the second layer. The interface between the first and second layers is capable of exhibiting electronically controlled ferromagnetism.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/661,908, filed Oct. 23, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/806,169, filed Nov. 7, 2017, now U.S. Pat. No.10,490,331, which is a divisional of U.S. patent application Ser. No.14/801,410, filed Jul. 16, 2015, now U.S. Pat. No. 9,852,835, whichclaims priority from U.S. Provisional patent application Ser. No.62/025,815, filed Jul. 17, 2014. The contents of these applications areincorporated herein by reference in their entirety.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grant no.W911NF-08-1-0317 awarded by the Army Research Office, grant no.FA9550-10-1-0524 awarded by the Air Force Office of Scientific Research,and grant nos. 1104191 and 1124131 awarded by the National ScienceFoundation. The government has certain rights in the invention.

TECHNICAL FIELD

The present invention is generally directed to oxide interfaces and morespecifically to oxide interfaces displaying electronically controlledferromagnetism.

BACKGROUND

The two-dimensional electron liquid that forms at the interface betweenthe two insulating non-magnetic oxides LaAlO₃ (“LAO”) and SrTiO₃ (“STO”)has drawn widespread attention due to its possession of a remarkablevariety of emergent behavior including superconductivity, strongRashba-like spin-orbit coupling, and ferromagnetism. See Ohtomo, A. &Hwang, H. Y., “A high-mobility electron gas at the LaAlO₃/SrTiO₃heterointerface,” Nature, 427: 423-426 (2004); Reyren et al.,“Superconducting interfaces between insulating oxides.” Science, 317:1196-1199 (2007); Ben Shalom et al., “Tuning Spin-Orbit Coupling andSuperconductivity at the SrTiO₃/LaAlO₃ Interface: A MagnetotransportStudy,” Phys. Rev. Lett., 104 (2010); Brinkman, A. et al., “Magneticeffects at the interface between non-magnetic oxides.” Nature Materials,6: 493-496 (2007).

Despite this interest, the existence and nature of magnetism instructures comprising a LAO/STO interface has remained controversial.Neutron reflectometry measurements by Fitzsimmons et al. on LAO/STOsuperlattices found no magnetic signatures; their measurementsestablished a bulk upper limit thirty times lower than what was reportedby Li et al. See Fitzsimmons et al., “Upper Limit to Magnetism inLaAlO₃/SrTiO₃ Heterostructures,” Phys. Rev. Lett., 107: 217201 (2011).Salman et al. reported relatively small moments from LAO/STOsuperlattices (˜2×10⁻³ μB /unit cell) using (3-detected nuclear magneticresonance. See Salman et al., “Nature of Weak Magnetism in SrTiO₃/LaAlO₃Multilayers,” Phys. Rev. Lett., 109: 257207 (2012). As a result, controlof such magnetism has remained elusive.

SUMMARY

An embodiment of the present invention provides an electronicallycontrollable ferromagnetic oxide structure that includes at least threelayers. The first layer comprises SrTiO₃ (STO). The second layer has athickness of at least about 4 unit cells and preferably not more thanabout 99 unit cells, the thickness being in a direction substantiallyperpendicular to the interface between the first and second layers. Thethird layer is in contact with either the first layer or the secondlayer or both, and is capable of altering the charge carrier density atthe interface between the first layer and the second layer. Theinterface between the first and second layers is capable of exhibitingelectronically controlled ferromagnetism.

Another embodiment of the present invention provides a method ofelectronically weakening or removing a ferromagnetic state at aninterface between a first layer and a second layer of a multi-layeredoxide structure. The method comprises establishing a voltage differencebetween the interface and a material in contact with at least one layerof the multi-layered oxide structure, wherein the voltage difference issufficient to increase the charge carrier density at the interface. Thefirst layer of the oxide structure comprises STO. The second layer has athickness of at least about 4 unit cells thick and preferably not morethan about 99 unit cells, the thickness being in a directionsubstantially perpendicular to the interface between the first andsecond layers. The interface between the first and second layers of theoxide structure is capable of exhibiting electronically controlledferromagnetization.

Another embodiment of the present invention provides a method ofelectronically establishing an anisotropic ferromagnetic statesubstantially in a direction {right arrow over (B)} at an interfacebetween a first layer and a second layer of a multi-layered oxidestructure. The method comprises establishing a voltage differencebetween the interface and a material in contact with at least one layerof the multi-layered oxide structure, wherein the voltage difference issufficient to decrease the charge carrier density at the interface. Thestep of establishing a voltage difference is performed while a magneticfield B substantially in a direction {right arrow over (B)} is presentat the interface between the first and second layers. The first layer ofthe oxide structure comprises STO. The second layer of the oxidestructure has a thickness at least about 4 unit cells thick andpreferably not more than about 99 unit cells, the thickness being in adirection substantially perpendicular to the interface between the firstand second layers. The interface between the first and second layers ofthe oxide structure exhibits substantially no ferromagnetizationimmediately prior to the step of establishing a voltage difference.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1B depicts a top gated oxide interface. It shows the effect ofapplying different voltages on the in-plane ferromagnetic state of anoxide interface.

FIG. 2 depicts an LAO/STO oxide interface with a top circular electrodeand an annular interfacial contact. It shows a voltage being appliedbetween the interface and the LAO layer via those contacts.

FIG. 3 depicts a phase diagram showing charge carrier density andtemperature dependence of a ferromagnetic state at an oxide interface.The phase diagram is a very rough approximation, and any numbers showntherein are not necessarily accurate.

FIG. 4 depicts a method of switching a memory state from state “0” tostate “1”.

FIG. 5 depicts a cross-bar array of memory devices having various memorystates, as well as word lines and bit lines.

DETAILED DESCRIPTION

The present invention provides for electronic control of ferromagnetismat oxide interfaces by making use of the fact that ferromagnetism atsuch interfaces depends strongly on charge carrier density at theinterface. The manuscript titled “Room-TemperatureElectronically-Controlled Ferromagnetism at the LaAlO₃/SrTiO₃Interface,” authored by Bi et al., available athttp://arxiv.org/ftp/arxiv/papers/1307/1307.5557.pdf, is herebyincorporated by reference in its entirety. The charge carrier density atan oxide interface can be controlled using a number of techniques,including but not limited to metallic gating, reorientable ferroelectricmaterials, electrolytes, polar adsorbates, self-assembled monolayers,and nanoscale control using conductive atomic force microscopylithography.

Under certain conditions described herein, depleting charge carriers(for example, electrons) from an oxide interface results in theformation of a ferromagnetic phase with defined domain walls, theferromagnetic phase being substantially in the plane of the interface,wherein the plane is not necessarily flat. Increasing charge carrierdensity at the interface by introducing additional charge carriers via,for example, electrical gating, results in the weakening and/or removalof the ferromagnetic state. The added charge carriers alignantiferromagnetically with the existing magnetic state, weakening it andpotentially removing it. Subsequent depletion of charge carriers resultsin a new, uncorrelated magnetic pattern at the interface. FIG. 1A showsan oxide interface displaying a non-ferromagnetic phase, and FIG. 1Bshows an oxide interface displaying a ferromagnetic phase, where V₁caused an increase in charge carrier density at the interface, and V₂caused a decrease in charge carrier density at the interface.

An “interface,” as referred to herein, includes a plane between twolayers in contact with each other, and further includes a thickness ofabout 4 unit cells, extending from the plane, into each layer. Theinterface also includes any passivation layer, such as TiO₂, whichterminates one or both surfaces of the two layers in or around the planeof contact.

Electronically Controllable Ferromagnetic Oxide Structure

An electronically controllable ferromagnetic oxide structure cancomprise, for example, a first layer and a second layer in contact witheach other forming an oxide interface between the two layers. Theinterface is preferably, but not necessarily, flat. The structure canfurther comprise a third layer in contact with at least one of the firsttwo layers. The third layer is capable of altering the charge carrierdensity at the oxide interface. An oxide interface capable ofelectronically controllable ferromagnetism can comprise, for example,the interface formed between two layers, one layer comprising STO andthe other comprising at least one of LAO, LaTiO₃, EuTiO₃, Al₂O₃, GaTiO₃,and/or LaMnO₃.

In an embodiment of the present invention an oxide interface is formedbetween two layers, one comprising LAO and the other one comprising STO.The STO layer can be grown, for example, on a substrate (for example, ona silicon substrate). The STO layer is preferably at least 10 nm thick,and is most preferably at least 100 nm thick. The LAO layer ispreferably at least about 4 unit cells thick and not more than about 99unit cells thick, and any value in between, and is most preferably atleast about 8 unit cells thick and not more than about 30 unit cellsthick, and any value in between. In other embodiments, the LAO layer canbe about 4, about 5, about 6, about 7, about 8, about 9, about 10, about11, about 12, about 13, about 14, about 15, about 16, about 17, about18, about 19, about 20, about 21, about 22, about 23, about 24, about25, about 26, about 27, about 28, about 29, about 30, about 40, about50, about 60, about 70, about 80, about 90 unit cells thick, or not morethan about 99 unit cells thick.

In this embodiment, LAO/STO interfaces can, for example, be fabricatedby depositing 12 unit cell thick LAO films on TiO₂-terminated [001] STOsubstrates using pulsed laser deposition with in situ high-pressurereflection high energy electron diffraction. The TiO₂ termination canserve as a passivation layer. Alternatively, a [111] or [110] STOsubstrate may be used. Before such laser deposition, low-miscut (lessthan about 0.1°) STO substrates are preferably used and are etched usingbuffered HF acid to keep the TiO₂-termination. Then the STO substratesare annealed at about 1000° C., preferably for several hours, so thatsubstantially atomically flat surfaces are created. During thedeposition, a KrF exciter laser beam with a wavelength of approximately248 nm is preferably used and is focused on a stoichiometric LAO singlecrystal target with energy density of about 1.5 J/cm² and each LAO unitcell is preferably deposited by about 50 laser pulses. It is preferableto use one of two different growth conditions for the substrate growthwhich involve temperature and chamber background partial oxygen pressureP(O₂). Growth condition (1) involves T=about 550° C. and P(O₂)=about10⁻³ mbar; growth condition (2) involves T=about 780° C. and P(O₂)=about10 ⁻⁵ mbar. For samples grown in condition (2), after deposition theyare preferably annealed at about 600° C. in P(O₂)=about 300 mbar forabout one hour, which helps to minimize oxygen vacancies. Oxide growthcan also, for example, be accomplished, for example, by molecular beamepitaxy, or by sputtering.

In another embodiment, a first layer and a second layer forming an oxideinterface can be in contact with a third layer that is capable ofaltering the charge carrier density at the interface, thus forming anelectronically controllable ferromagnetic oxide structure. The thirdlayer can be in contact with only one of the first and second layers, orthe third layer can be in contact with both the first and second layers.For example, the third layer may comprise a metallic electrode which isin contact with both of the layers forming an oxide interface.

The charge carriers can comprise, for example, electrons. The thirdlayer can comprise, for example, at least one of a metallic electrode, areorientable ferroelectric material, an electrolyte, a polar adsorbate,a self-assembled monolayer, and the tip of an atomic force microscopeprobe. The reorientable ferroelectric material can comprise, forexample, (Pb,Zr)TiO₃ (“PZT”).

In another embodiment, a first and second layer form an oxide interfaceand a third layer comprising a metallic electrode is in contact with atleast one of the first and second layers. The third layer can have avoltage applied to it to establish a voltage difference between theinterface and the at least one layer, thereby altering the chargecarrier density at the oxide interface. Alternatively or additionally, atip of an atomic force microscope probe, instead of a metallicelectrode, can be used to apply such a voltage difference. In anotherembodiment, a first and second layer form an oxide interface and a thirdlayer comprising a polar adsorbate and/or an electrolyte is in contactwith at least one of the first and second layers, thereby alteringcharge carrier density at the interface. In another embodiment, a firstand second layer form an oxide interface and a third layer comprising aferroelectric material (for example, PZT) is in contact with at leastone of the first and second layers. The ferroelectric material can bereoriented in order to alter the charge carrier density at theinterface.

In another embodiment, an oxide structure comprises an oxide interfaceformed between a first layer and a second layer, for example, a firstlayer of STO and a second layer of LAO, and further comprises a thirdlayer which comprises a top metallic electrode, the third layer being incontact with the LAO layer. The oxide structure is, for example,patterned with top circular electrodes and concentric arc-shapedinterfacial contacts, as shown in FIG. 2. The top circular electrodesare preferably deposited on the LAO layer on a surface substantiallyopposite to the LAO/STO interface via, for example, DC sputtering. Thearc-shaped interfacial contacts extend through the LAO layer to theinterface. They are preferably prepared by creating trenches that extendfrom a LAO surface substantially opposite to the LAO/STO interfacethrough the LAO layer to the interface, via, for example, Ar-ionmilling, followed by deposition of Au and Ti. Au and Ti need notnecessarily be used, but Au is convenient for the wire bonding processand Ti can be useful for Au adhesion. The amount of Au and Ti depositeddepends upon the thickness of the LAO layer. For example, about 4 nm ofTi and about 30 nm of Au can be deposited. In other embodiments, theamount of Ti deposited can range from about 1 to about 50 nm, and anyamount in between, and the amount of Au deposited can range from about10 to about 40 nm, and any amount in between. In other embodiments, Tineed not be deposited. A series of metallic circular top electrodes (forexample, about 4 nm Ti and about 40 nm Au) are preferably deposited on aLAO surface substantially opposite to the LAO/STO interface. In otherembodiments, the amount of Ti deposited can range from about 1 to about50 nm, and any amount in between, and the amount of Au deposited canrange from about 10 to about 40 nm, and any amount in between. In otherembodiments, Ti need not be deposited. The arc-shaped interfacialcontacts can have, for example, a width of about 20 μm and fixedseparation of about 50 μm to the edge of the circular top gates. Inother embodiments, arc-shaped interfacial contacts have a width rangingfrom about 10 to about 30 μm, and any value in between, and they havefixed separation of about 25 to about 75 μm to the edge of the circulartop gates, and any value in between. The entire oxide structure, whichcan be about 5 mm×about 5 mm×about 0.5 mm, and any value in between, canbe affixed to a ceramic chip carrier using, for example, silver paint.In other embodiments, an entire oxide structure ranges from about 1 toabout 10 mm ×about 1 to about 10 mm×about 0.1 to about 1 mm, and anyvalue in between. Electrical contacts to bonding pads on the device are,for example, made with an ultrasonic wire bonder using gold wires.

Method of Electronically Weakening or Removing a Ferromagnetic State atan Interface Between a First Layer and a Second Layer of a Multi-LayeredOxide Structure

In another embodiment, a method for electronically weakening or removinga ferromagnetic state at an interface between the first and secondlayers of a multi-layer oxide structure comprises applying a voltagedifference between the interface and a material in contact with at leastone layer of the multi-layered oxide structure to increase the chargecarrier density at the interface. For example, to increase electrondensity at an LAO/STO interface of an oxide structure, a top circularelectrode in contact with the LAO layer on a LAO surface substantiallyopposite to the LAO/STO interface can be grounded, and a voltage −V_(dc)can be applied to an annular interfacial contact which is deposited onthe LAO surface substantially opposite to the LAO/STO interface andwhich extends through the LAO layer to the. Note that this configurationis equivalent to grounding the LAO/STO interface and applying +V_(dc) tothe top electrode. Increasing V_(dc) leads to electron accumulation atthe LAO/STO interface, increasing the electron density, which can resultin the weakening or removing of a ferromagnetic state at the interface.When increasing the electron density at the interface, V_(dc) canpreferably be about 0.01 to about 15 volts, and any value in between,and can be most preferably about 0.02 to about 6 volts, and any value inbetween. In other embodiments, V_(dc) can be about 1, about 2, about 3,about 4, about 5, about 6, about 7, about 8, about 9, about 10, about11, about 12, about 13, about 14, or about 15 volts.

Method of Electronically Establishing an Anisotropic Ferromagnetic Stateat an Interface Between a First Layer and a Second Layer of aMulti-Layered Oxide Structure

In another embodiment, a method of electronically establishing ananisotropic ferromagnetic state substantially in a direction {rightarrow over (B)} at an interface between the first and second layers of amulti-layered oxide structure comprises applying a voltage differencebetween the interface and a material in contact with at least one layerof the multi-layered oxide structure to decrease the charge carrierdensity at the interface, where the interface between the first andsecond layers of the oxide structure exhibited substantially noferromagnetization immediately prior to the step of applying a voltagedifference. The interface preferably exhibited less ferromagnetizationthan a background magnetic field B described below. For example, todecrease electron density at an LAO/STO interface of an oxide structure,a top circular electrode in contact with the LAO layer on a surfacesubstantially opposite to the LAO/STO interface can be grounded, and avoltage −V_(dc) can be applied to an annular interfacial contact whichis also on the LAO layer on a surface substantially opposite to theLAO/STO interface. Note that this configuration is equivalent togrounding the interface and applying +V_(dc) to the top electrode.Decreasing V_(dc) depletes the interface of electrons, decreasing theelectron density, which can result in the formation of a ferromagneticstate at the interface. The decreasing of electron density at theinterface should be performed while a background magnetic field Bsubstantially in a direction {right arrow over (B)} is present at theinterface between the two layers to have the newly formed ferromagneticstate be substantially in the {right arrow over (B)} direction. Themagnetic field B is preferably about 0.01 oersted to about 100 oersted,and any value in between, and is most preferably about 0.02 oersted toabout 10 oersted, and any value in between. In other embodiments, themagnetic field B can be about 1, about 2, about 3, about 4, about 5,about 6, about 7, about 8, about 9, about 10, about 20, about 30, about40, about 50, about 60, about 70, about 80, about 90, or about 100oersted. When decreasing the electron density at the interface, V_(dc)is preferably about −0.01 to about −15 volts, and any value in between,and is most preferably about −0.02 to about −6 volts, and any value inbetween. In other embodiments, V_(dc) can be about −1, about −2, about−3, about −4, about −5, about −6, about −7, about −8, about −9, about−10, about −11, about −12, about −13, about −14, or about −15 volts.

Working Conditions

The above described methods are preferably performed at certain chargecarrier density ranges. FIG. 3 shows a phase diagram displaying sometemperature dependence and interfacial electron density dependence of anoxide interface's ability to display ferromagnetism. Note that FIG. 3does not precisely characterize this relationship and serves only as avery rough guide to the present discussion. The above described methodsare preferably performed using an oxide interface that is in phase B orC of the phase diagram. Phase B represents the ferromagnetic state. Thetrend shown in the diagram is that the higher the temperature of anoxide structure, the smaller the range of acceptable carrier densitiesfor ferromagnetism. At too high a temperature, no ferromagnetic stateexists. Thus, the temperature is preferably below about 450 Kelvin, andmore preferably below about 320 Kelvin. The temperature can also be anyvalue lower than 450 Kelvin. When charge carrier density is too low(i.e. in phase A of the phase diagram), ferromagnetism will probably notbe supported and decreasing charge carrier density will probably notlead to the formation a ferromagnetic state. Thus the LAO layer ispreferably at least about 4 unit cells thick to help ensure a sufficientnumber of charge carriers. The LAO layer can also be a larger unit cellthickness as described herein.

Additionally, there is a type of magnetic state that might not besusceptible to the methods described herein for weakening or removing aferromagnetic state. This magnetic state can be achieved when chargecarrier density is high and when the temperature is low (i.e. phase D ofthe phase diagram). When an oxide interface is in such a state,increasing charge carrier density might not weaken or remove themagnetic state. Thus the temperature is preferably at least about 40Kelvin. The temperature can also be higher than 40 Kelvin.

Memory Device

Materials with a ferromagnetic state can exhibit various domainstructures, with the local magnetization taking on one of severaldiscrete directions {right arrow over (M)}={right arrow over (M₀)},{right arrow over (M₁)}. . . {right arrow over (M_(n))}. Regarded as amemory device, these n different states can store log₂(n) bits ofinformation. An embodiment of the present invention allows for theformation of, and/or electronic control of, such ferromagnetic statesand can be employed towards a memory device. See FIG. 1.

FIG. 4 illustrates a method for reversible orientation of ferromagneticstates, based upon an embodiment of the present invention that comprisesmetallically top-gated LAO/STO oxide structures. Such an oxidestructure, which can serve as a memory device, can be fabricated, forexample, from a two-layer oxide structure that comprises an LAO/STOinterface by depositing a conducting top layer onto the LAO layer on asurface substantially opposite to the LAO/STO interface.

A magnetic bit state can be defined for the memory device, for example,according to a magnetic moment orientation of a ferromagnetic domainfound at the LAO/STO interface: the magnetic bit state can be said to bein state “0” when the magnetic moment is pointing in one directionsubstantially in the plane of LAO/STO interface, and it can be said tobe in state “1” when the magnetic moment is pointing substantially inthe opposite direction.

To change such a magnetic bit state from “0” (FIG. 4A) to “1”, apositive voltage V_(C) is first applied to the conducting top layer(where V_(C) is gauged relative to the voltage at the interface) toweaken or remove the ferromagnetic state corresponding to “0” byincreasing charge carrier density at the interface (FIG. 4B). Thevoltage V_(C) can be preferably about 0.01 to about 15 volts, and anyvalue in between, and can be most preferably about 0.02 to about 6volts, and any value in between. In other embodiments, V_(dc) can beabout 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8,about 9, about 10, about 11, about 12, about 13, about 14, or about 15volts.

Next, a small external magnetic field B_(o) substantially in a direction{right arrow over (B)} corresponding to “1” is applied (FIG. 4C), whichwill help to set the magnetization orientation in a later step. Thesmall external magnetic field applied can be applied globally and may,but need not, be confined only to the memory device being switched forpurposes of switching the magnetic bit state of the memory device.However, it may be desirable to so confine the magnetic field. The smallexternal magnetic field is preferably about 0.01 oersted to about 100oersted, and any value in between, and is most preferably about 0.02oersted to about 10 oersted, and any value in between. In otherembodiments, the magnetic field B_(o) can be about 1, about 2, about 3,about 4, about 5, about 6, about 7, about 8, about 9, about 10, about20, about 30, about 40, about 50, about 60, about 70, about 80, about90, or about 100 oersted.

Next, the voltage applied to the conducting top layer relative to theinterface is switched to a negative gate voltage V_(I) to form a newferromagnetic state at the LAO/STO interface by decreasing the chargecarrier density at the oxide interface. The voltage V_(I) is preferablyabout −0.01 to about −15 volts, and any value in between, and is mostpreferably about −0.02 to about −6 volts, and any value in between. Inother embodiments, V_(dc) can be about −1, about −2, about −3, about −4,about −5, about −6, about −7, about −8, about −9, about −10, about −11,about −12, about −13, about −14, or about −15 volts. The newferromagnetic state's magnetization orientation will be substantiallyaligned with {right arrow over (B)}, which corresponds with state “1”(FIG. 4D). The small external magnetic field can then be removed and thebit state “1” is stored (FIG. 4E). During this operation, the step ofswitching to a positive gate voltage to weaken or remove theferromagnetic state and the step of applying a small external magneticfield {right arrow over (B)} can be performed simultaneously or inreverse order, and those steps can overlap. Preferably, these steps areperformed before the step of decreasing the charge carrier density atthe oxide interface is performed.

To read out such a magnetic bit state, a variety of methods can be usedsuch as, for example, magnetic force microscopy, Hall sensor, and giantmagnetoresistance head.

Cross-Bar Array

Based on the above memory device concept, a cross-bar array of memorydevices can be formed (FIG. 5) which allows convenient electroniccontrol of a large number of memory devices. An embodiment of such across bar array comprises a plurality of memory devices comprisingelectronically controllable ferromagnetic oxide structures, each oxidestructure comprising at least three layers. The first layer comprisesSTO. The second layer is in contact with the first layer and comprises,for example, LAO. The second layer is preferably no less than about 4unit cells thick and not more than about 99 unit cells thick in adirection substantially perpendicular to the interface between the firstand second layers, and is most preferably no less than 8 unit cellsthick and not more than 30 unit cells thick. Other unit cell thicknessescan be used as described herein. A ferromagnetic state at the interfacebetween the first and second layers defines the magnetic bit state ofthe memory device. The third layer is capable of altering the chargecarrier density at the interface between the first and second layers andcan comprise, for example, a metallic electrode. Within the cross-bararray, memory devices can preferably be spaced between about 2 nm apartto about 300 nm apart, and any value in between, and can most preferablybe spaced between about 10 nm apart to about 100 nm apart, and any valuein between.

The embodiment further comprises a plurality of bit lines which aresubstantially parallel to one another and are substantially disposed ina first plane. The bit lines can be, for example, conductive. A bit linecan, for example, comprise a layer of a first material and a layer of asecond material that is different from the first material. For example,a bit line can comprise a first metallic layer and a second coatinglayer over the metallic layer. The third layer of each oxide structurecomprises at least a portion of at least one bit line such that, forexample, the third layer of an oxide structure comprises a portion of abit line comprising a metallic layer of the bit line and a coating layerof the bit line. The third layer of each oxide structure serves as ametallic electrode capable of altering the charge carrier density at theinterface between the first and second layers of the oxide structure.

The embodiment may further comprise a plurality of word lines that aresubstantially parallel to one another and are substantially disposed ina second plane, wherein the first plane is substantially parallel to thesecond plane, and each bit line is substantially perpendicular to eachword line. Furthermore, at least one of the layers of each oxidestructure is in contact with at least one word line. The word line can,for example, offer structural support and/or can serve as an electrode.

This embodiment allows for electronic control over memory devices thatcomprise, for example, an LAO/STO interface. It allows for, among otherthings, establishing a voltage difference between the LAO/STO interfaceand a bit line in contact with the LAO layer. Such a voltage differencecan be used to switch a memory state in the manner described above. In asimilar manner, a word line can additionally or alternatively be usedfor purposes of electronically controlling memory devices. Either orboth of a word line or a bit line can be alternatively or additionallyused for structural support for the cross-bar array. Thus, convenientelectronic control over a large number of memory devices can beachieved.

As used herein, the term “about” will be understood by persons ofordinary skill in the art and will vary to some extent depending uponthe context in which it is used. If there are uses of the term which arenot clear to persons of ordinary skill in the art given the context inwhich it is used, then “about” will mean up to plus or minus 10% of theparticular term.

All publicly available documents referenced herein, including but notlimited to U.S. patents, are specifically incorporated by reference.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the methods and compositionsof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1-27. (canceled)
 28. A method of controlling an electronicallycontrollable ferromagnetic oxide structure, the electronicallycontrollable ferromagnetic oxide structure comprising: (a) a first layerin contact with a second layer, wherein the second layer has a thicknessof at least about 4 unit cells, the thickness being in a directionsubstantially perpendicular to an interface between the first and thesecond layers, wherein the interface is defined by a plane between thefirst and second layers; and (b) at least one surface electrode and atleast one interfacial contact, the at least one surface electrode andthe at least one interfacial contact being in contact with at least oneof the first layer or the second layer, wherein: (i) the at least onesurface electrode and the at least one interfacial contact areconfigured to alter the charge carrier density at the interface betweenthe first and second layers, (ii) the at least one surface electrode isdeposited on the second layer on a surface spaced from the interface,and the at least one interfacial contact extends from the surface spacedfrom the interface through the second layer to the interface, and (iii)the interface between the first and the second layers is configured toexhibit electronically controlled ferromagnetism in response toalteration of the charge carrier density, the method comprising: (1)applying a positive voltage to the at least one surface electrode; (2)applying a magnetic field to the electronically controllableferromagnetic oxide structure; and (3) switching the positive voltageapplied to the at least one surface electrode to a negative voltage soas to cause the interface to switch between a ferromagnetic state and anon-ferromagnetic state.
 29. The method of claim 28, wherein thepositive voltage and the magnetic field are applied simultaneously. 30.The method of claim 28, wherein the positive voltage and the magneticfield are applied prior to a reduction in the charge carrier density atthe interface.
 31. The method of claim 28, wherein the magnetic field isapplied in a direction corresponding to a magnetic bit state definedaccording to a magnetic moment orientation of a ferromagnetic domain ofthe interface.
 32. The method of claim 28, wherein the first layercomprises SrTiO₃ and the second layer comprises at least one of LaAlO₃,LaTiO₃, EuTiO₃, Al₂O₃, GaTiO₃, or LaMnO₃.
 33. The method of claim 28,wherein the interface comprises a TiO₂-terminated SrTiO₃ surface. 34.The method of claim 28, wherein the at least one surface electrodecomprises at least one of Ti or Au.
 35. The method of claim 28, whereinthe at least one interfacial contact is an arcuate contact disposed soas to be arranged concentrically around at least a portion of the atleast one surface electrode.
 36. The method of claim 28, wherein the atleast one surface electrode comprises a plurality of metallic circulartop electrodes disposed in series.
 37. The method of claim 28, whereinthe at least one surface electrode is grounded.
 38. A method of alteringa ferromagnetic state at an interface of a multi-layered oxide structurecomprising at least a first layer and a second layer, the methodcomprising: establishing a voltage difference between the interface anda material in contact with at least one of the layers of themulti-layered oxide structure, the interface being between the first andsecond layers of the oxide structure and defined by a plane between thefirst and second layers, wherein: (a) the voltage difference issufficient to alter a charge carrier density at the interface betweenthe first and second layers of the oxide structure; (b) the second layerhas a thickness in a direction substantially perpendicular to theinterface between the first and second layers; (c) the oxide structurefurther comprises at least one surface electrode and at least oneinterfacial contact, the at least one surface electrode and the at leastone interfacial contact being in contact with at least one of the firstlayer or the second layer, and (d) the interface between the first andsecond layers of the oxide structure is capable of exhibitingelectronically controlled ferromagnetism, wherein the at least onesurface electrode is deposited on the second layer on a surface spacedfrom the interface, and the at least one interfacial contact extendsfrom the surface spaced from the interface through the second layer tothe interface.
 39. The method of claim 38, wherein: (a) the voltagedifference is about 0.01 to about 15 volts; and (b) the voltage appliedto the material in contact with the at least one layer is greater thanthe voltage applied to the interface.
 40. The method of claim 38,wherein the interface comprises a TiO₂-terminated [001] SrTiO₃ surface.41. The method of claim 38, wherein the first layer comprises SrTiO₃ andthe second layer comprises at least one of LaAlO₃, LaTiO₃, EuTiO₃,Al₂O₃, GaTiO₃, or LaMnO₃.
 42. The method of claim 39, wherein the atleast one surface electrode comprises at least one of Ti or Au.
 43. Themethod of claim 38, wherein the at least one interfacial contact is anarcuate contact disposed so as to be arranged concentrically around atleast a portion of the at least one surface electrode.
 44. The method ofclaim 38, further comprising grounding the at least one surfaceelectrode.
 45. The method of claim 38, wherein establishing the voltagedifference comprises: (a) grounding the at least one surface electrode,and (b) applying a voltage to the at least one interfacial contact so asto increase electron accumulation at the interface.